Spina bifida
The prevention of poliomyelitis and the low prevalence of other
Neurologic disorders
conditions leading to scoliosis mean that surveys of scoliosis
Spinal muscular atrophy
largely reflect the incidence and prevalence of AIS.3 The inci-
Cerebral palsy
dence of minor curves in the US population as assessed by x-ray
Polio
is 4 in 1000.6 The incidence of deformities that reach angles of 358
Neurofibromatosis
is 1 in 1000, and of deformities greater than 708, approximately 0.1
Myopathic disorders
in 1000.7 These larger curves occur predominantly in females.
Myotonic dystrophy
The reported prevalence of scoliosis ranges from 0.3 to 15.3%.8
Muscular dystrophy
Although the prevalence in the American population is 1.8%, if
Connective tissue disorders
minor curvatures (5â€“108) are included, the rate in adult females is
Marfan syndrome
closer to 10%.9,10 Scoliosis screening programs and early inter-
Rheumatoid disease
ventions are advocated to prevent the natural progression of the
Osteochondystrophies
disease. Evidence suggests a reduction in the incidence of uncor-
Achondroplasia/hypochondroplasia
rected major curves in adults, likely due to early diagnosis and
Osteogenesis imperfecta
intervention.11
Osteoporosis of pregnancy
Severe scoliosis is rare in parturients and this rarity probably
Infection
results from the relatively low prevalence of moderate to severe
Tuberculosis
curves in the population, because pregnancy is common in
Posttraumatic
women who have scoliosis. In a postal survey of women diag-
nosed with scoliosis in Minnesota, 72% of responders had been
pregnant an average of 2.8 times each.12 Most of these women
(68%) had idiopathic scoliosis with the majority adolescent-
onset disease. Their mean curve size was 378, and most of the
spinal curves (61%) were thoracic or thoracolumbar. Fifty-eight
percent of the patients had undergone spinal surgery for
scoliosis, which may account for the relatively small Cobb
angles in the group. Three reviews confirmed that pregnancy
is common in women with scoliosis.13,14,15 There were no
important differences between the groups with respect to the
likelihood of marriage, number of pregnancies, incidence of
gestational back complaints, or the requirement for cesarean
section (C/S).

Risk factors for curve progression
Progression of a curve is defined as an increase of 58 or more, as
measured by the Cobb method, over subsequent assessments.
Progression is most likely to occur in the rapid adolescent growth
phase in immature patients; in patients with larger curves (>208)
at the time of original diagnosis; and in patients with double
curves at presentation.16 There is a threefold increase in the risk
Figure 7.1 Cobb angle â€“ schematic representation. A line is drawn parallel to the
of progression if the initial curve is measured at >208 than if the
superior cortical plate of the proximal end vertebrae and to the inferior cortical
curve is <208, and thoracic curves are more likely to progress if
plate of the distal end vertebrae. A perpendicular line is erected to each of these
lines. The angle of intersection is the (Cobb) angle of the curve. (From Crosby, the Cobb angle is large (> 508).
E. T. Musculoskeletal disorders. In Chestnut, D. H. (ed.), Obstetric Anesthesia: Although it was once thought that there was little progression
Principles and Practice, 3rd edn. Philadelphia, PA: Elsevier Mosby, 2004, p. 859.) after skeletal maturity in untreated patients, observation over
decades has shown that moderate curves (60â€“808) increase an
average of 308.17 The natural history of the untreated severe
paralytic scoliosis as well as curves resulting from osteochondys-
curve is progression of the deformity over time, resulting in
trophies. Infectious causes of scoliosis, predominantly tubercu-
early death from cardiopulmonary failure.3,17,18 Long-term follow-
losis related, are reported primarily from underdeveloped
countries.4 Scoliosis may also result from vertebral fractures, up of patients with major, uncorrected curves demonstrated

130
Chapter 7

Figure 7.2 Cobb angle â€“ chest x-ray. Cobb angles are represented
on this x-ray of a young woman with a progressive spinal muscular
atrophy (Kugelberg-Welander syndrome) and a 708 thoracic curve.
Note should also be made of the rib separation on the right
hemithorax compared with the left. This patientâ€™s pelvic film is
detailed in Figure 7.6.

that the mortality rate was twice that of the general population, altered. This change is important if major neuraxial block is
and the average age at death was 46.6 years.19 considered, because the underlying structures no longer main-
tain the same relationships to surface landmarks.

Skeletal changes in idiopathic scoliosis
Indications for intervention and principles
The skeletal anatomic pathology that results from AIS is complex.
of corrective surgery
Deformation of vertebrae is present when Cobb angles are >408,
as are abnormal relationships between vertebrae, excess curva- The goal of surgery is to fuse the spinal curve and prevent pro-
ture in the frontal plane, loss of normal sagittal plane curves, and gression of the deformity. Modern surgical techniques consis-
rotation in the vertical axis.20 The vertebral bodies have shorter, tently yield a 50% reduction of the deformity, without excessive
thinner pedicles and laminae on the concave side and a narrower risk. The area fused should be kept as short as possible to main-
vertebral canal (See Figure 7.3). The transverse processes are tain the greatest number of mobile articulations, but enough of
anatomically abnormal and asymmetric in their spatial orienta- the spine must be fused to stabilize the deformity. The most
tion. The spinous processes are deformed and skewed from the common stabilization technique remains the posterior fusion
midline. and instrumentation. Common to all the techniques described
The rotatory component associated with the scoliotic curve is is the requirement for spinal instrumentation and extensive bone
such that the axial rotation of the vertebral body is typically into grafting in the axial spine (see Figure 7.5).
the convexity of the lateral curve, and the spinous process is Follow-up studies of patients who underwent early operative
rotated back into the concavity (see Figure 7.4A).21 As a result of corrections of severe scoliotic curves demonstrated either
the rotation of the vertebrae, the ribs on the side of the convexity improvement in lung volumes and function, or the progression
are pushed backward, producing a prominent posterior angle â€“ of the restrictive lung disease has been arrested postopera-
tively.22,23,24 Improved function was demonstrated when signifi-
the rib hump (see Figure 7.4B). The interlaminar space is shifted
more toward the curve convexity than is the spinal process, and cant reductions in moderate to severe curves were achieved
the usual anatomic relationship between these structures is with Harrington instrumentation or when thoracic kyphosis was

131
2 Musculoskeletal disorders

(a)

Figure 7.3 Scoliotic deformation of the vertebral body. The vertebra
diagrammed is from a spine with a moderate to severe right-sided curve. The
body has shorter, thinner pedicles on the concave (left) side and a narrower
vertebral canal. The transverse processes are abnormal and asymmetric in their
spatial orientation. The spinous process is deformed and skewed from the
midline. (See also Figure 7.7.)

normalized.25,26 Delaying correction until adulthood appears to
reduce the gains made in lung function, when compared with
results of earlier correction.27 Patients who undergo early instru-
mentation generally do not develop the cardiopulmonary com-
plications that afflict patients with severe and uncorrected
disease.22,23,27,28,29
Figure 7.4 Idiopathic scoliosis â€“ lumbar spine. (A) X-ray study of the lumbar
spine in a 26-year-old woman with idiopathic scoliosis. The spinous process and
Cardiopulmonary pathophysiology in idiopathic pedicles are rotated away from the curve convexity and into the concavity. The
scoliosis epidural space was entered easily by directing the needle about 158 off the
perpendicular at the skin level toward the convexity of the curve. (From Crosby,
Respiratory pathophysiology
E. T. Musculoskeletal disorders. In Chestnut, D. H. (ed.), Obstetric Anesthesia:
Scoliosis interferes with the formation, growth, and development
Principles and Practice, 3rd edn. Philadelphia, PA: Elsevier Mosby, 2004, p. 860.)
of the lungs.30 Because the number of alveoli increase greatly
(B) Rib hump â€“ schematic. As a result of rotation of the vertebrae, the ribs on the
between birth and age eight, the occurrence of scoliosis before side of the convexity are pushed backward, producing the prominent posterior
lung maturity reduces the number of alveoli formed. The pul- angle, the rib hump. The intercostal gap is increased in the hemithorax with the
monary vasculature forms in parallel with the alveoli and is like- rib hump. (See Figure 7.2.)
wise affected, resulting in increased pulmonary resistance,
pulmonary hypertension, and, in severe cases, right heart failure.
The pulmonary pathophysiology of scoliosis also includes the in dyspnea on exertion and reduced exercise capacity in the early
effects of the vertebral and ribcage deformity on the mechanical stages. Progression of the curve results in greater respiratory
function of the lung. The key findings that correlate with respira- compromise.
tory compromise are (1) a thoracic curve; (2) thoracic lordosis; Although the residual volume (RV) is generally not affected in
and (3) a ribcage deformity. The most common abnormality is a most patients with restrictive lung disease, functional residual
restrictive pattern of pulmonary dysfunction with a reduction in capacity (FRC) is decreased. If the FRC is sufficiently reduced,
lung volume and compliance. This pattern is seen in all patients airways may close during normal tidal breathing, resulting in
with thoracic curves > 658. Ventilatory reserve is limited, resulting ventilation/perfusion (V/Q) mismatch and arterial hypoxemia.

132
Chapter 7

(b)

Figure 7.4 cont.

Total lung capacity (TLC â€“ the volume of the lung at end-maximal
inspiration) and vital capacity (VC â€“ the volume that can be
exhaled from the lungs starting from maximal inspiration) are
also both reduced. Normal VC in adults is 70 to 80 ml/kg. When
VC is reduced to < 15 to 18 ml/kg, expiratory airflow may become
inadequate to produce an effective cough. Flow rates, as meas-
ured against lung volumes, provide a measure of the presence or
absence of airway obstruction. These ratios tend to be unaffected
in restrictive lung disease, implying that intrinsic airways disease
is not typically associated with scoliosis.
The work of breathing depends on many factors, the most
important of which are the stiffness of the lungs and chest wall
and the resistance to flow through the airways. In patients with
thoracic scoliosis, the chest wall is stiff, larger transpulmonary
gradients must be generated to achieve airflow and more work is
necessary to expand the lungs to any volume. The actual work
done is reduced if patients with scoliosis breathe more rapidly at
smaller volumes. However, a normal dead space in conjunction
with small-tidal-volume breathing results in increased wasted
ventilation. Increased ventilatory requirements may result in a
large increment in respiratory work and as the respiratory work Figure 7.5 Harrington rod instrumentation. X-ray of the lumbar spine in a
31-year-old woman with thoracolumbar scoliosis corrected with spinal
increases, the potential for respiratory failure increases. If the
instrumentation. There is rotation of the vertebrae into the curve (toward the
respiratory muscles are forced to work at a sustained intensity
rod), and extensive bone grafting is evident adjacent to the rod. Two lumbar
of > 40% of maximum, muscle fatigue and respiratory failure
interspaces are not involved in the fusion, L4â€“L5 and L5â€“S1. (From Crosby,
result.31
E. T. Musculoskeletal disorders. In Chestnut, D. H. (ed.), Obstetric Anesthesia:
Dyspnea on exertion occurs before the onset of alveolar hypo-
Principles and Practice, 3rd edn. Philadelphia, PA: Elsevier Mosby, 2004, p. 861.)
ventilation. The degree of spinal deformity usually correlates with
symptom severity. Cardiorespiratory symptoms are not common
with curves < 708. Dyspnea is more common as the deformity exceeds 25%, and children with congenital heart disease have an
increased incidence of scoliosis.35 However, most patients with
exceeds 1008, and alveolar hypoventilation occurs when angles
exceed 1208. In younger patients with moderate thoracic scoliosis AIS do not have congenitally abnormal hearts. The cardiovascular
(25 â€“70%), impaired exercise capacity is usually due to decondi- abnormality that is most commonly associated with scoliosis
tioning and lack of regular aerobic exercise, not to intrinsic venti- results from the restrictive pulmonary defect. The consequences
latory impairment.32 Patients with severe scoliosis (curve > 908) of impaired lung development and alveolar hypoxemia are
are more likely to experience sleep-breathing abnormalities, increased pulmonary vascular resistance, pulmonary hyperten-
night-time hypoxemia, and daytime hypercapnia.33 sion and right ventricular (RV) hypertrophy. Permanent changes
of the pulmonary vasculature are common with curvatures > 658.
Cardiovascular pathophysiology A brief discussion of the parturient with pulmonary hyperten-
Scoliosis and cardiac anomalies may have a common embryolo- sion follows; the evaluation and care of patients with pulmonary
gic etiology.34 In AIS, the incidence of mitral valve prolapse hypertension is discussed in detail in Chapters 1 and 3. Pulmonary

133
2 Musculoskeletal disorders

hypertension complicating respiratory disease is generally with scoliosis. Diaphragmatic weakness or paralysis attributable
defined as a resting mean pulmonary artery pressure (PAP) to the underlying disorder can further compromise VC. If ribcage
exceeding 20 mmHg.36 This differs from the definition of primary expansion is limited by neuromuscular involvement, respiratory
pulmonary hypertension, which is a pressure of 25 mmHg at rest function is severely compromised and a restrictive pattern of lung
or 30 mmHg during exercise. Pulmonary hypertension at rest or disease develops. With advancing gestation, encroachment of the
with exercise occurs in many patients with a moderately advanced expanding uterus further compromises lung function and
deformity long before the onset of detectable right heart failure.7 respiratory insufficiency results. Hypoxemia may be present for
Pulmonary hypertension is mainly attributable to increases in prolonged periods before the onset of hypercapnia. Pulmonary
pulmonary vascular resistance (PVR) resulting from chronic vasoconstriction, hypertension, and RV failure occur owing to the
alveolar hypoxia (PaO2 < 60 mmHg), hypoxic pulmonary vasocon- same etiologic considerations as for idiopathic scoliosis. A pri-
striction, and anatomic vascular alteration. Fixed pulmonary mary myocardial impairment may also be superimposed on the
hypertension, unresponsive to supplemental oxygen (O2) therapy, acquired cardiovascular derangements in conditions such as
muscular dystrophy and Marfan disease.42
carries a grave maternal prognosis during pregnancy and is an
indication to recommend termination of pregnancy.37,38,39 An example of a neuromuscular disorder is spinal muscular
Patients with pulmonary hypertension have a limited ability to atrophy (SMA), a genetic syndrome characterized by progressive
increase cardiac output (CO) with activity.40 Because of the lim- degeneration of spinal anterior horn cells (see Chapter 10). After
ited CO, tachydysrhythmias are not well tolerated and may pro- Duchene dystrophy, SMA is the most common serious neuro-
duce marked systemic hypotension. If the RV fails in the presence muscular disease of childhood, and after cystic fibrosis, it is the
of pulmonary hypertension, left ventricular (LV) filling decreases second most common autosomal recessive disorder (estimated
incidence 1 in 10 000).43 The clinical spectrum is wide and ranges
and low-output failure and sudden death may occur. Death may
be caused by sudden changes in venous return to the right ven- from severe weakness and death during infancy to minimal weak-
tricle, with acute LV failure, ischemia, and dysrhythmias. ness with little impact on life expectancy. Marked scoliosis is a
Hypoxia, due to the increased O2 consumption associated with common manifestation of more severe expressions of SMA, espe-
labor as well as the underlying pulmonary dysfunction, further cially in patients with onset early in childhood. Historically, these
compromises cardiovascular function by increasing PVR and RV women would have been discouraged from reproducing, but with
afterload, and decreasing CO.41 Tiny pulmonary emboli may be advances in medical care, longer-term survival is expected and
fatal in established pulmonary hypertension because cardiac many of these women ultimately choose to have a partner and
reserve may be so marginal that even a small decrease in vascular have children. Case reports and case series detailing the repro-
compliance may fatally compromise ventricular function. ductive experiences of women with SMA and the care provided to
them are increasing.44
Scoliosis associated with neuromuscular disease:
Cardiopulmonary manifestations
Interaction of pregnancy with scoliosis
The pathophysiologic sequelae of scoliosis developing conse-
Impact of pregnancy on the spinal deformity
quent to a primary neurologic or myopathic disorder differ from
those of idiopathic scoliosis. Abnormal respiratory function Pregnancy may exacerbate both the severity of spinal curvature
results not only because of the skeletal deformity of scoliosis but and the cardiorespiratory abnormalities in patients with uncor-
also because of abnormalities in the central control of respiration rected scoliosis. The factors that predict curve progression are the
and in supraspinal innervation of muscles. Abnormal respiratory same in parturients as they are in nonpregnant women. Thus, a
function results also from loss of muscle function due to lesions of young, skeletally immature woman with scoliosis would be at
the motor neurons and peripheral nerves or as a result of myo- particular risk for curve progression during pregnancy. Curves
pathy. Further compromise may result from impairment of the that are <258 or curves that have been stable before the preg-
nancy do not, as a rule, progress during pregnancy.45,46,47 More
airway defense mechanisms caused by loss of control of the
pharynx and larynx, by ineffective cough mechanisms, and by severe curves and those that have not yet stabilized may progress,
although curve progression during pregnancy is uncommon.14,15
infrequent or reduced large breaths. Recurrent aspiration pneu-
monitis results from these compromised airway-protective There is no evidence of curve progression in women treated with
reflexes. The prognosis for the patient with scoliosis caused by bracing or surgery as adolescents, who subsequently become
pregnant.14,15 In two studies,14,15 women were followed for an
neuromuscular disease is determined predominantly by progres-
sion of the primary disorder and is worse than that for idiopathic average of 22 and 17 years. Maternal morbidity and mortality
scoliosis. have been linked to the severity of the curve, but the true correla-
The neuromuscular disorders usually involve both inspiratory tion appears to be with the degree of functional impairment
present before pregnancy.48 Patients with severe curves (Cobb
and expiratory muscles, resulting in moderate to severe decreases
angle !908) but good cardiopulmonary function tolerate preg-
in inspiratory capacity and expiratory reserve volume. Until the
nancy well.49 The incidence of gestational back pain is higher
diseases are well advanced or until a significant degree of thoracic
scoliosis is superimposed, FRC remains normal. Hypoventilation than expected in patients with uncorrected scoliosis, but not in
those who have undergone spinal fusion.50
is a prominent feature of the neuromuscular disorders associated

134
Chapter 7

Effect of pregnancy on the cardiopulmonary
Table 7.2 Risk factors for ventilatory failure in parturients
pathophysiology of scoliosis
with neuromuscular scoliosis
When evaluating the parturient with significant cardiopulmonary
Elevated PaCO2
disease, an attempt has to be made to distinguish the signs and
Bilateral diaphragmatic impairment
symptoms that are consistent with normal pregnancy and advan-
Extensive intercostal muscle weakness
cing gestation from those that may herald deterioration in a
Vital capacity <1.0 liter
chronic maternal condition. For example, although most parturi-
Cobb angle >1008
ents complain of dyspnea by the middle of the third trimester,
exercise testing shows no deterioration in exercise response dur-
ing moderate activity. A pathologic deterioration in respiratory
unmedicated parturient increases by a further 75 to 150% in the
function is associated with a significant decrease in exercise tol-
first stage and by 150 to 300% in the second stage. These levels
erance. Two features help distinguish physiologic from patholo-
gic dyspnea.51,52 Physiologic dyspnea tends to begin earlier in may be either unattainable or unsustainable by the scoliotic
parturient with restrictive lung disease, and respiratory insuffi-
pregnancy and often reaches a plateau or improves as term
ciency or failure may result.
approaches. It is rarely extreme, and patients can usually main-
In parturients with neuromuscular scoliosis, decreased lung
tain daily activities. The dyspnea of cardiopulmonary decom-
volumes with advancing pregnancy result in increased V/Q mis-
pensation is progressive, becoming more severe as gestation
matching, decreased arterial O2 content, and carbon dioxide
advances and physiologic demands are increasing. If dyspnea is
retention. These effects may be especially marked during sleep
extreme, has a limiting impact on normal activity, occurs at rest
because of a further reduction in lung volumes due to loss of
or with minimal exertion, or is associated with a cough, mater-
nal cardiorespiratory decompensation should be ruled out.51 muscle tone during sleep and enhanced cephalad shift of the
diaphragm during supine positioning. The upper airway resist-
Dyspnea that is acute in onset or progressive and intractable,
ance rises during pregnancy because of mucosal hyperemia,
especially if coupled with other signs and symptoms (orthopnea,
increasing secretions, and occasional development of nasal
paroxysmal nocturnal dyspnea), is more likely to represent car-
polyps. These changes predispose the patient to snoring and
diopulmonary disease.
obstructive sleep apnea.54 Weakness of the muscles that stabilize
The thoracic cage expands in circumference during normal
the upper airway is common in diffuse muscle disorders: the
pregnancy as a result of increases in both anteroposterior and
weakness may increase the incidence, severity, and maternalâ€“
transverse diameters. Little potential exists for further thoracic
fetal implications of the sleep apnea that develops. All of these
cage expansion during inspiration. Inspired volumes in the term
factors increase alveolar hypoxia and worsen pulmonary hyper-
pregnant woman are largely attributable to diaphragmatic excur-
tension.36 A stage of dyspnea on exertion as a prelude to more
sion. If the chest cage is fixed by scoliosis, the diaphragm is
severe incapacity is seen only rarely in neuromuscular scoliosis.
entirely responsible for all increments in minute ventilation
This is perhaps because neuromuscular dysfunction has possibly
(MV). As the enlarging uterus enters the abdominal cavity in
long since rendered such exertions untenable. Risk factors for
midgestation, diaphragmatic activity is constrained. Functional
ventilatory failure during pregnancy have been identified (see
residual capacity decreases to 70% (supine) to 80% (upright) of
Table 7.2).55 The use of noninvasive (negative pressure) ventila-
nonpregnant values by term gestation. Closing capacity (CC) is
tion to reduce dyspnea and improve respiratory function has
also reduced. Even greater than anticipated decreases in FRC and
been reported.43
CC may be seen in patients with scoliosis, resulting in V/Q mis-
Cardiac output increases about 40% by the end of the first
match and reduced arterial O2 content.
trimester and is 50% above nonpregnant levels by the third tri-
Minute ventilation increases by 40 to 50% during pregnancy
mester; both heart rate and stroke volume increase to augment
and the increase is primarily a result of increased tidal volume
CO. In scoliotic parturients who already have increased PVR, it
(VT), with respiratory rate relatively unchanged. In the scoliotic
may not be possible to achieve this increase in CO without further
patient with restrictive lung disease, such rises in VT may not be
increments in vascular pressures, increasing RV afterload. This
possible, and the increased MV is achieved via increased respira-
may place an intolerable load on the RV, precipitating right heart
tory rate. Increased respiratory rate increases both wasted venti-
failure with low CO leading to poor myocardial perfusion and
lation and the work of breathing; respiratory failure may result.